As part of utilizing biomass energy, attempts have been made to decompose cellulose or hemicellulose as main components of plants to obtain ethanol. The thus obtained ethanol is planned to be mainly used as fuel such as part of automotive fuel or a gasoline alternative.
The main components of plants include cellulose (a polymer of glucose as a C6 sugar containing 6 carbon atoms), hemicellulose (a polymer of a C5 sugar containing 5 carbon atoms and a C6 sugar), lignin, and starch. Ethanol is produced by fermentation action of microorganisms, such as yeast, using, as a raw material, sugars such as C5 sugars, C6 sugars, and oligosaccharides as complexes of them.
The industrial use of the following three methods is being contemplated to decompose cellulosic biomass such as cellulose or hemicellulose into sugars: 1) a hydrolysis method utilizing the oxidation power of a strong acid such as sulfuric acid; 2) an enzymatic decomposition method; and 3) a method utilizing the oxidation power of supercritical water or subcritical water. However, it is difficult to practically use the acid decomposition method 1) from an economical viewpoint. This is because an added acid acts as an inhibitor of yeast fermentation, and therefore absolutely needs to be neutralized after decomposition of cellulose or hemicellulose into sugars and before alcoholic fermentation of the sugars, and the neutralization treatment is costly. The enzymatic decomposition method 2) can be performed at ordinary temperature and constant pressure, but no effective enzyme has been found. Even if an effective enzyme is found, it is expected that the production cost of the enzyme will be expensive. Therefore, from an economical viewpoint, there seems to be no prospect for actually using the enzymatic decomposition method on an industrial scale.
PTL 1 discloses, as the method 3) for hydrolyzing cellulosic biomass into sugars with supercritical water or subcritical water, a method for producing a water-insoluble polysaccharide by bringing a cellulose powder into contact with pressurized hot water at 240 to 340° C. to hydrolyze cellulose. PTL 2 discloses a method in which biomass cut into small pieces is hydrolyzed with hot water pressurized to a saturated water vapor pressure or higher at 140 to 230° C. for a predetermined time to decompose/extract hemicellulose, and is then hydrolyzed with pressurized hot water heated to a decomposition temperature of cellulose or higher to decompose/extract cellulose. PTL 3 discloses a method for producing glucose and/or a water-soluble cello-oligosaccharide, in which cellulose having an average degree of polymerization of 100 or higher is subjected to a contact reaction with supercritical water or subcritical water at a temperature of 250° C. or higher but 450° C. or lower and a pressure of 15 MPa or higher but 450 MPa or lower for 0.01 second or longer but 5 seconds or shorter, and is then cooled and hydrolyzed by contact with subcritical water at a temperature of 250° C. or higher but 350° C. or lower and a pressure of 15 MPa or higher but 450 MPa or lower for 1 second or longer but 10 minutes or shorter.
PTL 4 discloses a method for inexpensively and easily producing sugars using a wood-based plant as a raw material while reducing the amounts of energy and water to be used for hydrolyzing polysaccharides, in which treatment water for use in hydrolysis treatment contains acidic water obtained after sugars are recovered from a sugar-containing hydrolysis extract obtained by hydrolysis treatment of the wood-based plant. According to PTL 4, the pH of the treatment water for use in hydrolysis is preferably 2.6 to 2.9.
PTL 5 discloses a sugar production method in which polysaccharides are decomposed by a hydrothermal reaction in hot water containing an organic acid such as formic acid, acetic acid, or oxalic acid at a temperature of 120 to 300° C. and a pressure of 0.2 to 100 MPa. PTL 5 discloses that the decomposition time of polysaccharides can be reduced by adding an organic acid.
PTL 6 discloses a lignocellulose pretreatment method in which lignocellulosic biomass is hydrolyzed in dilute sulfuric acid at 140 to 220° C. for 3 to 20 minutes, the hydrolysate is then subjected to solid-liquid separation and thus separated into a primary saccharified solution and a solid (dewatered cake), the solid is mixed with hydrated lime and heated at 90 to 150° C. for to 120 minutes, and the lime-treated solid is then enzymatically hydrolyzed using cellulase to obtain a secondary saccharified solution. PTL 6 also discloses that a hydrated lime-containing liquid separated by subjecting the solid after lime treatment to solid-liquid separation is used for neutralization of the primary saccharified solution containing dilute sulfuric acid.